I have a day & night heater/ ac unit. When ever it feels like the fan comes on and blows for a few minutes then shuts off again. Sometimes it goes for a week without doing it, then other times it does it every few minutes. I have disconnected the thermostat completely and that didn't do anything. The compressor outside doesn't come on nor does the heat, it just blows air. The diagnostic light inside the unit flashes normally . Saying that unit is working properly and no previous errors. The unit is only 6 years old. Any ideas as to what it could be? model # c8mpv075f14c1

I assume on the control board there are 2 relays, one for cooling air flow and one for heating air flow. A gremlin is causing the relay (either one) to close. To close the relay requires a signal from the microprocessor. The problem could be a sick microprocessor or other component on the control board or possibly something flaky in your house power to the furnace/air handler.

I would turn off the power to the air handler then check all 120 volt wiring paying close attention to the ground (bare wire) and neutral (white wire, usually) connections. Check the wiring from and at the furnace back to the circuit breaker panel. Double check the house ground at the circuit breaker panel and the neutral/ground bond to the power company neutral/ground. Possibly you have a floating ground.

If the problem still persists, check the control board for cold solder joints. I will try and find details on your specific model and add more later.